This is a continuation proposal for study of patients with intact A-V conduction and intraventricular conduction defect and for study of patients with evidence of sinus node dysfunction. Patients are detected by screening of inpatient and out-patient electrocardiograms. Patients are evaluated clinically, electrocardiographically, and electrophysiologically. Electrophysiological studies include recording of His potentials, measurements of pacing responses and refractory periods and evaluation of sinus mode function utilizing atrial pacing and extra-stimulus techniques. Patients are then followed prospectively in conduction disease clinics, with specific goals of detecting significant bradyarrhythmia and sudden death. Portable tape recorder monitoring is used to increase sensitivity of bradyarrhythmia detection. It is proposed that initial evaluation will show delineation of high risk groups (as determined by the subsequent follow-up). For example, H-V prolongation might correlate with subsequent development of symptomatic trifascicular block, or prolonged sinus recovery time, with subsequent symptomatic sinus node dysfunction. All initial and follow-up data is computerized, allowing analysis with rapid information retrieval, multi-variate analysis, and life table methods. BIBLIOGRAPHIC REFERENCES: Wu D., Denes P., Dhingra R.C., Amat-y-Leon F., Wyndham C.R., Chuquimia R., Rosen K.M.: Electrophysiological and clinical observations in patients with alternating bundle branch block. Circulation 53: 456, 1976. Dhingra R.C., Denes P.,Wu D., Wyndham C.R., Amat-y-Leon F., Towne W.D., Rosen K.M.: Prospective observations in patients with chronic bundle branch block and marked H-V prolongation. Circulation 53: 600, 1976.